An organism’s genome contains many variable number tandem repeats (VNTRs).…

The idea

Variable number tandem repeats (VNTRs) are loci where a short base sequence is repeated end to end. The repeat sequence is identical between individuals, but the number of times it is copied varies, creating different alleles. Many VNTRs occur in non-coding DNA; because these often do not alter polypeptides, changes in repeat number are usually not selected against, allowing many alleles to accumulate. A person inherits one allele at each locus from each parent. While unrelated people might share a repeat number at a single locus, testing multiple independent loci multiplies the separate probabilities. This makes a match at every locus vanishingly improbable, except in monozygotic twins who share identical DNA.

What earns the marks

• Describing a VNTR as a short base sequence repeated in tandem at a locus.
• Stating that it is the number of repeats, not the repeat sequence, that varies between individuals.
• Explaining that many VNTRs are non-coding, so changes in repeat number often do not affect the phenotype and are not selected against.
• Explaining that loci are inherited independently, so probabilities at each locus multiply, making a chance match across many loci vanishingly small.
• Identifying monozygotic twins as the exception because their DNA is identical.

Common mistakes and how to put them right

• Describing VNTRs as genes, or saying the repeats code for a protein.

• Saying the base sequence of the repeat varies between people, when it is the number of copies of that repeat that varies.

• Writing that every person's VNTR pattern is unique without excluding identical twins.

• Adding the probabilities from separate loci instead of multiplying them, which makes a chance match look more likely.

• Forgetting that each VNTR locus carries two alleles, one inherited from each parent.

Exam tips

• Put the word 'number' in your answer; the variation is in the number of tandem repeats, and answers that only say the sequence 'varies' rarely score.
• When asked why fingerprinting is reliable, name more than one locus and say the probabilities multiply, rather than just asserting that everyone is different.
• If a question supplies a probability for each locus, multiply across the loci rather than adding them, and write the product out to show your working.